1. Field of the Invention
The present invention relates to the field of optical amplifiers, and, more particularly, relates to the field of optical amplifiers comprising a length of optical fiber having an active dopant which fluoresces in response to pump light.
2. Description of the Related Art
Erbium-doped fiber amplifiers (EDFAs) are used extensively in commercial optical communication systems, both as all-optical repeaters and pre-amplifiers. After traveling through long lengths of communication fiber (typically several tens of kilometers), the information-encoded signals are strongly attenuated by fiber loss, and it is the role of the erbium-doped fiber amplifier (EDFA) to amplify the signals to a reasonable power level. EDFAs have now reached the point where they have been optimized with respect to their energy efficiency to minimize their requirement for pump power (which is costly). EDFAs have further been optimized with respect to their noise performance such that noise figures approaching the best-case theoretical limit of 3 dB are now possible. The gain flatness of optimized EDFAs now exceeds a few tens of dB over tens of nanometers of bandwidth. EDFAs can now be designed so that their gain depends very little on the polarization of the input signals.
One area of EDFA research that is still very active is the gain bandwidth. The gain bandwidth parameter is important because it ultimately dictates the number of signals of different wavelengths that can be amplified by a given EDFA. The broader the bandwidth is, the larger the number of individual signals that can be amplified, and therefore the higher the bandwidth (bits of information per unit time) that can be carried by a single fiber. Because the host affects the spectroscopy of the erbium ions, a number of fiber host materials, including silica, fluorozirconate glasses, and chalcogenides, have been and continue to be investigated in an attempt to identify a host that will provide a larger gain bandwidth for the 4I13/2→4I15/2 transition of Er3+. In silica-based glasses, the bandwidth is generally divided into what are known as the C-band and the L-band. In approximate terms, the C-band is the portion of the optical spectrum below about 1,565 nanometers, while the so-called L-band is the portion of the optical spectrum above about 1,565 nanometers. In silica-based fibers, the total bandwidth of the combined C and L-bands is approximately 80 nanometers, although this figure has only been attained so far by concatenating two EDFAs. (See, for example, Y. Sun, et al., 80 nm ultra-wideband erbium-doped silica fibre amplifier, Electronics Letters, Vol. 33, No. 23, November 1997, pp. 1965-1967.) The situation is similar with a fluorozirconate host. (See, for example, S. Kawai, et al., Wide bandwidth and long distance WDM transmission using highly gain-flattened hybrid amplifier, Proceedings of Optical Fiber Communication OFC'99, Paper FC3, February 1999, pp. 56-58.) In tellurite glass fibers, the total bandwidth is also around 80 nanometers, but it can be accomplished with a single fiber. (See, for example, Y. Ohishi, et al., Optical fiber amplifiers for WDM transmission, NTT R & D, Vol. 46, No. 7, pp. 693-698, 1997; and Y. Ohishi, et al., Gain characteristics of tellurite-based erbium-doped fiber amplifiers for 1.5-μm broadband amplification, Optics Letters, Vol. 23, No. 4, February 1998, pp. 274-276.
FIG. 1 illustrates an exemplary standard EDFA configuration 100 comprising an erbium-doped fiber (EDF) 110. Optical signals are input to the erbium-doped fiber 110 via a first optical isolator 120 and a wavelength division multiplexing (WDM) coupler 122. An optical pump signal from an optical pump source 124 is also input to the erbium-doped optical fiber 110 via the WDM coupler 122. The amplified output signals from erbium-doped optical fiber 110 are output through a second optical isolator 126. The optical isolators 126, 120 are included to eliminate backward reflections into the erbium-doped fiber 110 from the output port and to eliminate backward reflections from the erbium-doped fiber 110 to the input port, respectively. The erbium-doped optical fiber 110 can be pumped in the forward direction, as illustrated in FIG. 1, or in the backward direction (not shown) or in both directions. Because of the broad nature of the fiber gain medium, the configuration of FIG. 1 produces gain over a large bandwidth. For example, erbium-doped tellurite fibers and erbium-doped chalcogenide fibers have been used in the configuration of FIG. 1. As set forth in Y. Ohishi, et al., Gain characteristics of tellurite-based erbium-doped fiber amplifiers for 1.5-μm broadband amplification, Optics Letters, Vol. 23, No. 4, February 1998, pp. 274-276, gain bandwidths of around 80 nanometers have been produced using a tellurite fiber.
When the fiber host is a silica-based glass, gain cannot be provided over the whole bandwidth (approximately 1,525 nanometers to approximately 1,610 nanometers) with a single fiber. Instead, gain needs to be produced over two adjacent spectral regions, and then the outputs resulting from the gain are combined. A generic method for achieving broader gain bandwidth is to use hybrid amplifiers, in which two or more amplifiers made of different hosts are concatenated. The amplifiers are designed such that they provide gain spectra that complement each other, thus producing a larger overall gain bandwidth than either one of them. This method was successfully demonstrated with a silica-based EDFA followed by a fluoride-based EDFA, which produced a 0.5-dB-bandwidth of 17 nanometers. (See, P. F. Wysocki, et al., Dual-stage erbium-doped, erbium/ytterbium-codoped fiber amplifier with up to +26-dBm output power and a 17-nm flat spectrum, Optics Letters, Vol. 21, November 1996, pp. 1744-1746.) More recently, a similar concept was applied to two fluoride-based EDFAs. (See, Y. Sun, et al., 80 nm ultra-wideband erbium-doped silica fibre amplifier, Electronics Letters, Vol. 33, No. 23, November 1997, pp. 1965-1967.)
FIG. 2 illustrates an exemplary configuration 200 having two EDFAs 210, 220. One EDFA (the lower EDFA 210) is designed to amplify the C-band (from approximately 1,525 nanometers to approximately 1,565 nanometers), and the other EDFA (the upper EDFA 220) is designed to amplify the L-band (approximately 1,565 nanometers to approximately 1,620 nanometers). The two EDFAs 210, 220 advantageously include respective pump sources (not shown) which are coupled to the erbium-doped fibers using respective WDM couplers, as illustrated in FIG. 1. The input signals, which have different wavelengths λi spaced apart by a certain amount, are split into the two branches with a WDM coupler 230 and the amplified output signals are combined in an output coupler 232. An input optical isolator 240 and an output optical isolator 242 operate as described above. Because of the WDM coupler 230, signals with wavelengths less than approximately 1,565 nanometers are coupled into the lower branch to propagate to the C-band EDFA 210, and signals with wavelengths greater than approximately 1,565 nanometers are coupled into the upper branch to propagate to the L-band EDFA 220. (In practice, there is a narrow guard band between the C-band and the L-band to avoid overlapping signals in the two arms.) For example, a silica-based EDFA can be designed to have an L-band with a gain spectrum that is flat within 0.5 dB over the 1,568-nanometer to 1,602-nanometer range. The gain flatness is partly achieved, for example, by selecting the proper fiber length or with the use of filters. Both methods are well known in the art.
The C-band EDFA 210 and the L-band EDFA 220 can be made from the same erbium-doped fiber, or of different fibers, or of different host materials. The C-band and L-band EDFAs 210, 220 may differ in their respective designs, particularly with respect to pump wavelength, pump configuration and fiber length.
Generally, the upper limit of the L-band EDFA 220 is approximately 1,610 nanometers. There is a substantial effort in the research community to push this limit by adjusting the host material. The difficulty in further extending this limit resides in the presence of signal excited-state absorption (ESA) above around 1,620 nanometers for tellurite glass and above around 1,610 nanometers for silica glass. The ESA constitutes an undesirable signal loss mechanism. Based on these results, the current bandwidth record for silica-based (Y. Sun, et al., cited above) and fluoride-based EDFAs (S. Kawai, et al., cited above) is around 80-85 nanometers (i.e., similar to that of tellurite-based EDFAs as disclosed in the two Y. Ohishi, et al., articles cited above).